WASHINGTON, D.C. — Populations around the world are aging as mortality and fertility rates fall. While increased longevity represents a remarkable achievement of modern medicine and public health, the steep decline in fertility over the past decades is a cause for concern.

The collapse in birth rates can be attributed to powerful structural factors — such as urbanization, education gains and women's increased participation in the labor market — as well as the perception that childbearing is more manageable with a smaller number of children. As a result, governments' efforts to reverse the trend have fallen short.